The present invention relates to a crossed field, re-entrant beam amplifier. Such amplifiers generally have within a vacuum enclosure a substantially cylindrical cathode provided with two supports which ensure its fixing to the enclosure and a delay line with direct or backward wave propagation and which surrounds most of the cathode.
A first wave guide is connected to one of the ends of the delay line and supplies to it the microwaves to be amplified. A second wave guide connected to the other end of the delay line collects the amplified microwaves. The two ends of the delay line are separated by a so-called sliding zone.
An electrical field is established between the cathode and the delay line and a magnetic field is established in a direction perpendicular to the electrical field, in accordance with the axis of the cylinder constituting the cathode.
The operation of crossed field re-entrant beam amplifiers is explained by the interaction between the electron cloud from the cathode and the direct or backward microwaves propagated along the delay line. For certain values of the electric and magnetic fields, this electron beam forms a space charge arm which rotates around the cathode at a speed close to that of the microwaves. In this arm a first and larger part of the electrons passes from the cathode to the delay line to which the electrons transfer their potential energy and thus ensure the amplification of the signal. A second part of the electrons returns to the cathode and thus ensures its heating by bombardment. Finally, a third part of the electron is not collected and constantly rotates about the cathode, which gives rise to the term re-entrant beam tubes.
Crossed field, re-entrant beam amplifiers have the advantage of being compact and light weight, of having a high efficiency and of requiring only a single high voltage which has not been regulated with a high degree of precision, as is the case with the high voltages supplying travelling wave tubes. This high voltage establishes the electrical field between the cathode and the delay line. The energy due to the bombardment of the returning electrons is generally sufficient to heat the cathode.
The problem which arises is that the operating frequency band of crossed field re-entrant beam amplifiers is too narrow, being only approximately 10% about the central frequency.